Harvesters such as windrowers, tractors, and forage harvesters have to balance functionality with weight distribution. On one hand, the harvester needs to include a hitch at the front of the vehicle so that it can attach a header or some other harvesting equipment. On the other hand, the harvesting equipment is very heavy when attached on the front of the vehicle. If the vehicle is operating with a header attachment, the shock absorbers must accommodate weight distributed on the front end of the vehicle. If the header is not attached to the vehicle, the weight distribution of the vehicle is altered dramatically on most current vehicle designs such that the front of a harvester may be elevated over a horizontal or substantially horizontal plane. This elevation makes it difficult to attach a header.
Typical construction for such vehicles include front ground wheels mounted on the frame at fixed angles parallel to each other and parallel to a center line of the frame and rear ground wheels mounted on a respective caster. Each of the front ground wheels is typically driven by a respective drive motor which allows variable speed in both the first and second directions such that steering of the tractor is effected by a differential in speed between the front wheels with the rear wheels following the steering in a castering action. A pair of dampers or shock absorbers are each connected between a bracket on a rear axle of the frame and a lever in the caster plane of the caster so as to provide a damping force tending to restrict rotation of the respective second wheel about the respective vertical pivot axis with the damping force at a maximum value when the rear wheels are parallel to the center line in either the forward or reverse directions.
Such hydrostatically driven vehicles used primarily for swathing are commonly used and readily available. The vehicles typically carry at a forward end on suitable sprung supports a header for cutting standing crop with the crop being transported on the header to a suitable discharge location generally centrally of the vehicle for forming a swath in the field from the cut crop.
Such vehicles generally include a pair of front wheels just behind the header which are fixed to the frame of the vehicles so that they remain at an angle parallel to each other and parallel to a center line of the tractor. The tractor is supported at the rear end by a pair of caster wheels which are attached to a cross axle pivotally connected to the frame at a center horizontal pivot point, commonly known as a walking beam. The axle is typically supported relative to the ground with the caster wheels at the ends of the rear axle so that the wheels trail behind a vertical pivot mount for the wheels.
The front wheels only are driven and are driven in a manner which allows full control in the forward and reverse directions so that steering is effected by a differential speed between the two front wheels with the rear wheels following in the conventional castering action.
It is also known that such vehicles can travel more effectively at high speed when reversed in direction so that the driven wheels are at the rear and the caster wheels at the front. The caster wheels of course rotate through 180° to trail behind the vertical pivot which is now forward of the caster wheels as the tractor moves at relatively high speed in this reversed direction.
Maintaining the desired or proper ride height at the rear end of the vehicle is an inherent problem on vehicles of the above type. In some vehicles, the rear end of the vehicle rises dramatically during rapid deceleration while the vehicle is traveling in the forward direction, thereby placing excessive loads on the front of the vehicle. In some vehicles, the rear end of the vehicle lowers or squats dramatically during removal of the header, thereby placing excessive loads on the rear of the vehicle.
Many such vehicles use as the rear suspension of the rear caster wheels air bags (e.g., shock absorbers) as the springing medium, which may be effective when a header is mounted on the front of the vehicle and the vehicle has a higher percentage of weight on the front axle than on the rear axle. However, manual adjustments by the operator of the air bags is necessary when the operator is preparing to remove the header. Such manual adjustments necessitate that the operator exit the vehicle to insert lockout pins into the air bags when removing the header to restrict the rear end of the vehicle from squatting excessively when the weight of the header is removed from the front of the vehicle.
In addition, if the operator is switching from one header to another (e.g., a 16 foot disc head to a much heavier 40 foot draper head), the operator must manually adjust the volume and/or pressure of air in the air bags in order to compensate for the additional ballast that is required to balance the vehicle for the larger head. This activity of manually inserting or removing the lockout pins and/or manually adding air to the air bags can be frustrating and time consuming. The conventional rear suspension systems can also be inconsistent in terms of the ride height due to changes in temperature. For example, if the air bags are properly inflated in the morning, the air inside will expand as the temperature rises, causing the rear end of the vehicle to rise further and necessitating that the operator exit the cabin to manually deflate the air bags. The opposite occurs when operating into the night (e.g., the air bags lose pressure as the temperature decreases, necessitating that the operator manually inflate the air bags). Thus, conventional rear suspension systems require an extensive amount of manual intervention and adjustment from the operator.
For example, FIGS. 1-7 show side, perspective and detailed views of an exemplary conventional windrower 10 having a rear suspension system 12. The windrower 10 generally includes front wheels 14, 16 rotatably mounted to a frame 18, and the rear suspension system 12 mounted to the frame 18. The windrower 10 includes a cabin 20 configured and dimensioned to receive an operator, and having a plurality of controls for operation of the windrower 10, such as controlling a header (not shown) attachable to an attachment mechanism 22 at the front of the windrower 10, controlling movement of the windrower in a forward direction 24, and controlling movement of the windrower in a reverse direction 26. On each side of the windrower, the rear suspension system 12 includes a damper or shock absorber 28a, 28b (e.g., a shock or fixed hydraulic actuator) and an air bag or shock absorber 30 mounted to a rear axle 32a, 32b for regulating positioning of each caster 34, 36 upon which respective caster wheels 38, 40 are mounted. A ballast box 52 is part of the main frame 18 at the rear end of the windrower 10 and provides balance to the rear portion of the windrower 10. A height 60 measured between the bottom of the rear end (e.g., the ballast box 22) and the ground 56 defines the ride height of the windrower 10.
FIGS. 2-7 show detailed views of the rear suspension system 12. The shock absorber 30 includes a top section 42 that is positioned above and receives therein a bottom section 44. The shock absorber 30 includes a dampening element (e.g., a spring, air, or the like) disposed within the shock absorber 30. The top and bottom sections 42, 44 move relative to each other as the dampening element absorbs motions of the casters 34, 36. As shown in FIG. 4, the shock absorber 30 includes a slidable lockout pin 46 disposed below the bottom section 44 with a fastener pin 48 that maintains the desired position of the lockout pin 46. FIG. 4 shows the lockout pin 46 in the extended position, allowing the shock absorber 30 to provide suspension action. FIG. 5 shows the lockout pin 46 in the inserted position to prevent suspension action from the shock absorber 30 when removing the header. As noted above, adjustment of the position of the lockout pin 46 requires manual intervention from the operator, resulting in delay in operation of the windrower 10.
As shown in greater detail in FIGS. 6 and 7, the rear suspension system 12 includes two suspended left-hand and right-hand suspension assemblies. Each assembly includes a rear axle 32a, 32b operably attached to a respective caster 34, 36 through an absorber system which includes a mounting bracket 50a, 50b and a shock or fixed hydraulic actuator 28a, 28b. The casters 34, 36 are typically operably connected to a rear wheel assembly that includes a wheel and tire (e.g., wheels 38, 40) fixed to the bottom portion of the caster 34, 36 and allow some pivotal movement of the rear wheel and rear tire about a vertical axis that coincides with the attachment to the bottom portion of the casters 34, 36. The rear suspension system 12 can be mechanically attached to the frame 18 of the windrower 10. The right and left hand axles 32a, 32b are operably attached to the frame 18 by a plurality of fastening elements, which in this configuration, allow for a suspended left and right handed axle 32a, 32b. 
FIG. 1 shows both wheels 38, 40 in contact with the ground 56. However, when the header is mounted to the windrower 10 or the windrower 10 undergoes rapid deceleration while moving in the forward direction 24, the rear end of the windrower 10 can lift the wheels 38, 40 and the rear suspension system 12 off the ground 56, thereby increasing the height 60. When the header is removed from the windrower 10 or the windrower 10 undergoes rapid deceleration while moving in the reverse direction 26, the rear end of the windrower 10 can squat or lower to the ground 56, thereby decreasing the height 60. Such changes in the ride height 60 impart excessive forces on the rear suspension system 12 and the front wheels 14, 16, and generally necessitate manual intervention from the operator to correct based on the particular operation of the windrower 10.